Chucks which are commonly used in high speed dental handpieces for releasably holding dental burs in place are generally of the collet type (either pushout or drawback collets) or the spring grip type. In either case, a separate tool such as a wrench or push rod is usually required to open, close, or tighten the chuck. The necessity for such a separate tool can be inconvenient and it will be readily appreciated that a chuck that does not require such a tool will be advantageous.
A wrenchless chuck mechanism is disclosed in Austrian Pat. No. 354,612. The mechanism includes a drawback collet axially movable within the head of a dental handpiece. Forward axial movement to open the collet is effected by direct downward force exerted against the top of the head, which force is transmitted to the collet forcing it axially downward against the return force of a surrounding coil spring. When such external force is removed, the spring pushes the collet back to its closed position securing a bur shank within its sidewalls. The force required to open this collet is not insignificant; in practice it is usually applied by turning the handpiece over and pushing the top of the head downward against a surface such as a table top. This can be damaging to the instrument as well as inconvenient to the user.
German Offenlegungsschrift No. DE 3129286 discloses other forms of wrenchless collet chucks. In one embodiment (see FIG. 2) the top of the head is rotatable through a nearly 360.degree. arc to open and close a drawback collet. In another embodiment (FIG. 11) the top of the head is slidable in forward-reverse directions perpendicular to the axis of the collet, thus actuating a ball and cam combination to effect axial movement of the collet against a return spring. In addition, the German publication discloses other embodiments which operate by directly applied force without mechanical advantage essentially similar to the operation of the device shown in the Austrian patent.